International Space Station Environmental Control and Life ...

45th International Conference on Environmental Systems ICES-2015-094 13-17 July 2015, Seattle, Washington

International Space Station Environmental Control and Life

Support System Mass and Crewtime Utilization In

Comparison to a Long Duration Human Space Exploration

Mission

Robert M. Bagdigian1

NASA Marshall Space Flight Center, Huntsville, Alabama 35812

Jason Dake2

NASA Johnson Space Center, Houston, Texas, 77058

Gregory Gentry3 and Matt Gault4

The Boeing Company, Houston, TX 77058

Over the last two-and-a-half decades, the International Space Station’s (ISS) Environmental Control and

Life Support System (ECLSS) has grown and evolved in size, complexity, and capability. The functions that it

performs today are many of those that will need to be performed in the future aboard spacecraft and habitats

that will enable long duration human exploration missions to destinations beyond low earth orbit. Regardless

of the particular deep space destination, it is widely accepted that highly reliable ECLS systems that depend

minimally on expendable equipment will be required. An important question, particularly in today’s fiscally-

constrained environment, is how well suited is the ISS ECLSS suite of technologies to meeting the needs of

future missions? To help begin answering this question, the maintenance history of the ISS Water Recovery

and Oxygen Generation Systems has been surveyed. Equipment mass utilization rates, achieved hardware

operating lifetimes, and crewtime spent on maintenance tasks have been tallied to provide a surrogate measure

of reliability. These data are also compared to notional targets for a hypothetical three-year Mars mission.

Nomenclature

AES = Advanced Exploration Systems

ARFTA = Advanced Recycle Filter Tank Assembly

ATV = Automated Transfer Vehicle

CDRA = Carbon Dioxide Removal Assembly

CO2 = Carbon Dioxide

CR = Catalytic Reactor ORU

ECLS = Environmental Control and Life Support

ECLSS = Environmental Control and Life Support

System

DA = Distillation Assembly

DMSD = Dimethylsilanediol

FCA = Firmware Controller Assembly

FCPA = Fluids Control and Pump Assembly

GS = Gas Separator ORU

1 Chief, Environmental Control and Life Support Development Branch, NASA Marshall Space Flight Center,

Mailcode ES62, Huntsville, AL, 35812 2 International Space Station Environmental Control and Life Support System Manager, NASA Johnson Space Center,

Mailcode EC3, Houston, TX 77058 3 International Space Station Environmental Control and Life Support Technical Lead, 3700 Bay Area Blvd, Mailstop

HB2-40, Houston, TX 77058 4 International Space Station Logistics and Maintenance, 3700 Bay Area Blvd, Houston, TX 77058.

H2 = Hydrogen

H2 = Hydrogen ORU

H2O = Water

H2O = Water ORU

ISS = International Space Station

IX = Ion Exchange Bed ORU

kg = kilogram

MCV = Microbial Check Valve ORU

MLS = Mostly Liquid Separator

MTBF = Mean Time Between Failure

N2 = Nitrogen ORU

NASA = National Aeronautics and Space

Administration

O2 = Oxygen

International Conference on Environmental Systems

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O2 = Oxygen ORU

OGA = Oxygen Generation Assembly

OGS = Oxygen Generation System

ORU = Orbital Replacement Unit

P = Pump ORU

PC = Process Controller ORU

PCPA = Pressure Control and Pump Assembly

psia = pounds per square inch absolute

PS = Pump Separator ORU

PSM = Power Supply Module

R&R = Removal and Replacement

RFTA = Recycle Filter Tank Assembly

RSA = Rotary Separator Accumulator

SF = Separator Filter ORU

SPA = Separator Plumbing Assembly

UPA = Urine Processor Assembly

USOS = United States Orbital Segment

WD = Water Delivery ORU

WPA = Water Processor Assembly

WRS = Water Recovery System

WS = Water Storage ORU

WSTA = Wastewater Storage Tank Assembly

WW = Wastewater ORU

I. Background

An important objective of the National Aeronautics and Space Administration (NASA) strategic plan1 is to

“expand human presence into the solar system and to the surface of Mars to advance exploration, science, innovation,

benefits to humanity, and international collaboration”. To meet this objective, NASA is identifying and assessing

mission concepts that can be combined into an evolvable campaign of progressively more demanding missions that

incrementally demonstrate capabilities that will be needed to enable human exploration of the Mars surface2. Success

in achieving demanding campaign objectives within fiscally-constrained environments will depend heavily on

focusing investments on filling capability gaps that can’t be filled with today’s state-of-the-art, flight-proven systems

and technologies. The International Space Station (ISS) environmental control and life support system (ECLSS)

performs many of the functions that are expected to be needed in vehicles and habitats supporting an evolvable Mars

campaign. Most likely, among those functions will be the need to reliably recycle oxygen and water in order to reduce

the mass and volume burdens placed on transportation systems. The ISS Oxygen Generation System (OGS) and

Water Recovery System (WRS) have been recycling oxygen and water since 2007 and 2008, respectively. Since

activation, in addition to enabling sustained ISS crew operations, these systems have been providing valuable insights

into the challenges with long duration oxygen and water recycling in an operational, human-occupied, spacecraft

environment. They are also providing a first glimpse into the readiness to proceed to the much bolder and challenging

step of sending humans to Mars and returning them safely. This paper provides an initial, top-level assessment of the

life cycle mass and maintenance time devoted to OGS and WRS, both of which provide a surrogate indication of

reliability, and compare hardware operating lifetimes achieved to date to a representative Mars mission scenario.

II. Mars Reference Missions

Two key mission characteristics that largely determine the ECLSS capabilities that will be needed are the total

amount of time that a crew will need to be supported and whether that support will need to be provided in a micro-

gravity or reduced-gravity (surface) environment3. Strongly influencing the manner by which those capabilities can

be provided are the typical constraints on mass, volume, power, combined with unique deep-space exploration realities

that prevent quick crew return to earth in emergency situations4. These characteristics and constraints are described

for a number of candidate Mars exploration mission concepts in the Mars Design Reference Architecture (DRA) 5.0,

the latest in a series of NASA Mars reference missions5. Although it does not represent a formal plan for the human

exploration of Mars, DRA 5.0 does provide a vision of potential approaches of how various exploration systems could

be used to implement the first human landing on Mars. In-space transportation capabilities combined with Earth-Mars

alignment phasing yield Mars mission profiles that are generally grouped into two distinct classes of potential round-

trip Mars missions: opposition-class missions, which are also commonly referred to a short-stay missions, and

conjunction-class missions, referred to as long-stay missions.

Short-stay, opposition-class missions are typified by relatively short duration stays (typically 30 to 60 days) on

Mars and one relatively long transit leg (either outbound to Mars or inbound back to Earth). As a class, they generally

have the highest propulsive energy requirements. A working timeline presented in DRA 5.0 for an opposition-class

mission includes an outbound crew transit of 174 days, a crew stay on the surface of Mars of 60 days, and a return

crew transit of 400 days. An important characteristic of such a mission, particularly with respect to ECLSS, is its

combined 574 days of crew time spent transiting to and from earth in the micro-gravity environment of deep space.

Alternatively, short-stay missions which substitute 60 days of orbital exploration of the Mars moons Deimos and

Phobos, would result in 634 days that crew members spend in a micro-gravity environment.


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Long-stay, conjunction-class missions are characterized by long-duration stay-times on Mars (as much as 550

days) and moderately-long transit times. As a class, conjunction missions require the lowest amounts of propulsive

energy. A working timeline for a conjunction class mission in DRA 5.0 includes 220-day outbound and return transit

legs separated by a 539-day Mars surface stay. Alternatively, crew could spend the entire stay-time engaged in the

orbital exploration of Deimos and Phobos, resulting in nearly 1000 days of crew time spent in micro-gravity.

Surface missions have two particular characteristics that can substantially influence the functions needing to be

provided by an ECLS system and the design of the system to perform those functions. The gravitational force on

Mars is roughly one-third that on Earth and would be sufficient to employ simpler techniques for phase separations

than the centrifugal and membrane-based diffusion techniques used in the micro-gravity environment on the ISS.

Gravitational forces on Phobos and Demos are roughly 1/1000th that on Earth and would likely require the same or

similar phase separation techniques employed on the ISS. The second surface characteristic is the potential availability

of in-situ resources from which useable oxygen and water could be extracted to meet crewmember metabolic needs.

In a Mars surface architecture in which in-situ resource utilization (ISRU) is used to produce oxygen for ascent stage

propulsion, the incremental amount of oxygen that would also be needed to support crew metabolic needs would be

relatively small in comparison, perhaps making it unnecessary for a Mars surface habitat located in the vicinity of an

ISRU-based propellant production plant to include its own oxygen recycling capability. Since the trade space for

ISRU applications in a Mars surface architecture remains large, the focus of this study has been limited to the micro-

gravity transit phase. This paper uses a cumulative 1000-days of crew support in a micro-gravity environment,

comparable to that envisioned in DRA 5.0 for a conjunction-class, orbital exploration of Phobos and Deimos, as the

basis for comparison to ISS WRS and OGS equipment capabilities.

III. International Space Station Maintenance Data

All on-orbit ISS maintenance activities are logged in the ISS Maintenance Data Collection. The collection

provides a record of scheduled and unscheduled maintenance activities associated with ISS hardware, typically linked

down to the maintainable item level and tracked by both part number and serial number. For WRS and OGS

equipment, this level of tracking typically equates to the ORU-level. For each maintenance activity, the scheduled

service or unscheduled discrepancy that triggered the activity is identified and the corresponding corrective action is

briefly described. Time (hours) spent or allocated to each activity is recorded separately for each crewmember that

participated in the activity for each day that the activity was in progress. Time includes that which was allocated for

procedure review, locating and collecting required equipment and tools, executing the procedure or activity, etc.

Maintenance methods are categorized as follows:

Removal and Replacement (R&R): The item being maintained is removed from its operational location within the

higher level system and replaced with a separate, functionally equivalent item. This method is typically used to

restore system functionality after a hardware item has failed unexpectedly or has reached the end of its operational

life.

Remove and Reinstall: The hardware being maintained is removed from its operational location within the higher

level system to facilitate crew member inspection or troubleshooting. Upon completion of the inspection or

troubleshooting, the hardware item is reinstalled into its original operating location and returned to service.

Repair: The hardware being maintained is either removed from its operational location or left in place while

crewmembers complete actions to repair (return to functional condition) the item. Upon completion of the repair,

the hardware item is returned to service.

Troubleshooting: Crewmembers perform investigative procedures intended to obtain information needed to

understand the cause(s) of anomalous performance and determine suitable corrective actions.

Inspection/Service: Crewmembers observe hardware items to obtain information needed to understand its current

condition or perform routine procedures needed to sustain the items’ proper functionality and performance and

protect it from a functional failure.

Cleaning: Crewmembers perform routine cleaning of hardware items (typically air filters) needed to sustain proper

system functionality and performance.

The ISS Maintenance Data Collection has been used as the reference data used throughout this study. Tabulated

crew-times that are reported in this paper reflect how the activities are recorded in the collection. In some cases, as


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the understanding of system operation and failures have evolved over the life the WRS and OGS, the categorization

of maintenance activities has also evolved accordingly. Since the intent of this study was to provide a high-level

survey of maintenance trends, rather than a detailed accounting of maintenance records, data in the collection has been

used as-is.

IV. International Space Station Water Recovery System

The International Space Station (ISS) Water Recovery System includes two main assemblies. The Urine Processor

Assembly (UPA) accepts pretreated urine collected from the crew and processes it through a vapor compression

distillation process. Distillate that is produced is combined with humidity condensate that is collected from the ISS

cabin atmosphere; the combined distillate and humidity condensate is then purified to potable water quality

specifications via the Water Processor Assembly (WPA) which, through a sequence of purification steps, purifies the

water to meet potable water quality specifications.

A simplified functional schematic of the UPA is shown in Figure 1. The UPA is packaged into seven Orbital

Replacement Units (ORUs). Pretreated urine is delivered to the UPA either from the United States Orbital Segment

(USOS) Waste and Hygiene Compartment (outfitted with a Russian urinal) or via manual transfer from the Russian

urine container (called an EDV). The urine is temporarily stored in the Wastewater Storage Tank Assembly (WSTA).

Figure 1. Urine Processor Assembly Schematic (through October 2011 shown on left, current shown on right)

The Fluids Control and Pump Assembly (FCPA) includes a four-tube peristaltic pump that moves urine from the

WSTA into the Distillation Assembly (DA), recycles the concentrated waste from the DA into the Advanced Recycle

Filter Tank Assembly (ARFTA) and back to the DA, and pumps distillate from the DA to the WPA. The DA is the

heart of the UPA, and consists of a rotating centrifuge where water is evaporated from the recirculated urine/brine at

low pressure. The vapor is compressed and subsequently condensed on the opposite side of the evaporator surface to

conserve latent energy. A rotary lobe compressor within the DA provides the driving force for the evaporation and

compression of water vapor. At the end of each brine concentration cycle, a crew member removes the ARFTA from

the rack and transfers the concentrated brine within it into a Russian EDV tank or Rodnik tank using a Russian

compressor and hose assembly for ultimate disposal. The ARFTA is then refilled with pretreated urine, which allows

the process to repeat. The ARFTA has less capacity (approximately 22 L) than the original 41 liter Recycle Filter Tank

Assembly (RFTA) which was manually removed by the crew and replaced with an empty assembly at the end of each

concentration cycle. The capability to fill and drain the ARFTA on ISS avoids the costly resupply penalty associated

with returning, ground-servicing, and re-launching each RFTA. The Pressure Control and Pump Assembly (PCPA)

includes another four-tube peristaltic pump which provides for the removal of non-condensable gases and water vapor

from the DA. Liquid cooling of the pump housing promotes condensation, thus reducing the required volumetric

capacity of the peristaltic pump. Gases and condensed water are pumped to the Separator Plumbing Assembly (SPA),

which recovers and returns water from the purge gases to the product water stream. A Firmware Controller Assembly

(FCA) provides the command control, excitation, monitoring, and data downlink for UPA sensors and effectors.

A simplified schematic of the WPA is provided in Figure 2. The WPA is packaged into 15 ORUs. Wastewater

composed of cabin humidity condensate, distillate from the UPA, and Sabatier product water is delivered to the WPA

and temporarily stored in a bellows tank within the Waste Water ORU (WW). The bellows maintains a slight positive


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pressure to push water and gas into the centrifugal Mostly Liquid Separator (MLS) within the Pump/Separator ORU

(PS). Gas is removed from the wastewater by the MLS and passes through the Separator Filter ORU (SF) where odor-

causing contaminants are removed from entrained air before its return to the cabin. Degassed water is pumped through

the Particulate Filter ORU (PF), followed by two Multifiltration Beds (MFB) where inorganic and non-volatile organic

contaminants are removed. The Sensor ORU (S) located between the two MFBs determines when the first bed is

saturated based on conductivity. At that point, the first MFB is removed, the second MFB is moved up to the first

position, and a fresh MFB is installed in the second position. Effluent from the second MFB enters the Catalytic

Reactor ORU (CR), where low molecular weight organics not removed by the MFB sorbents are oxidized in the

presence of oxygen, elevated temperature, and a catalyst. A regenerative heat exchanger recovers heat from the

catalytic reactor effluent water and returns it to the reactor’s inlet. The Gas Separator ORU (GS) removes excess

oxygen and gaseous oxidation by-products from the process water and returns it to the cabin. The Reactor Health

Sensor ORU (RHS) monitors the conductivity of the reactor effluent as an indication of whether the organic load

coming into the reactor is within the reactor’s oxidative capacity. Finally, the Ion Exchange bed ORU (IX) removes

dissolved products of oxidation and adds iodine for residual microbial control. The water is subsequently stored in

the Water Storage ORU (WS) and delivered on-demand to users via a pump and accumulator within Water Delivery

ORU (WD). The WPA is controlled by a process controller (PC) that provides the command control, excitation,

monitoring, and data downlink for WPA sensors and effectors. An Oxygen Filter ORU protects CR components from

particulates and a Microbial Check Valve ORU (MCV) protects product water from microbial contamination through

the WPA’s recycle line.

Figure 2. Water Processor Assembly Schematic

The logistical mass benefit of recycling water can be seen clearly in Figure 3 by comparing the mass of water

recycled to the mass of hardware that has been used to perform the recycling. The initial mass of the WRS when it

was launched in 2008 was 1385 kg (3048 lb). This mass includes the combined mass of all installed UPA and WPA

processing and storage tank ORUs, two process controllers, rack outfitting equipment (including an avionics air

assembly, rack power control module, and smoke detector assemblies), structure (including two standard ISS racks,

shelves, and brackets), and interconnecting cables and hoses. Through March 20, 2015 the system had produced

22,350 kg (49,170 b) of recycled potable water. Since the beginning of 2013, the average daily potable water

production rate has been approximately 12.7 kg/day (28 lb/day), corresponding roughly to a 3.4-person water

processing rate. Overall water recovery efficiency has been approximately 88%. Recycling this amount of potable


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water has required equipment to be replaced at various times due to failures, life limits being reached or exceeded, the

original logistical approach of returning UPA RFTAs to the ground for servicing, and the replacement of the fleet of

RFTAs with two ARFTAs in 2011. Since system activation, 2225 kg (4896 lb) of equipment has been replaced,

including 941 kg (2070 lb) in response to failures and 1284 kg (2826 lb) due to consumption of planned expendable

items such as UPA RFTAs, WPA MFBs, etc. The combined original system mass and the cumulative mass of

equipment that has since been replaced is 3608 kg (7938 lb). Normalizing this mass of hardware utilized with the

mass of water produced yields an overall equipment (original non-recurring system plus recurring replacements)

utilization mass of 0.16 kg of equipment per kg of water produced. The recurring portion of this mass utilization rate

is 0.10 kg of equipment per kg of water produced over the entire operating life of the WRS, and 0.08 kg of equipment

per kg of water produced since the UPA RFTAs were replaced with ARFTAs.

Figure 3. Water Recovery System Life Cycle Mass (through March 20, 2015)

In addition to impacting logistical mass, equipment failures and the need to periodically replace limited life and

expended ORUs have manifested themselves in the amount of crew time that has been dedicated to maintaining

WRS operability. The record of crew time applied to WRS is shown in Figure 4.


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Figure 4. Crew Time Applied to Water Recovery System Maintenance (through March 20, 2015)

In the nearly 6.5 years since activation, about 320 crew-hours have been recorded against WRS maintenance

tasks. When normalized against the water production data shown in Figure 3, this equates to approximately 14.3

crew-hours per 1000 kg (6.5 crew-hours per 1000 lb) of water recycled. Three quarters (247 crew-hours) were

dedicated to the removal and replacement of ORUs. The remaining quarter included tasks recorded as

troubleshooting (33 hours), repair (21 hours), inspection and servicing (16 hours), and removal and reinstallation (<1

hour).

Inspection of Figures 3 and 4 reveals some key information, much of which is being used by NASA to target

investments that will benefit the ISS program in the near-term and deep space missions in the longer term6,7. For

example, 36% of logistical mass and of crew time have been associated with the handling and disposal of urine

brine. Initial use of UPA RFTAs represents the largest WRS logistical mass (668 kg, 1470 lb), despite the fact that

their usage was curtailed in 2011 in lieu of ARFTAs. However, the logistics mass benefit of switching to ARFTAs

has come at the expense of crew-time, as crew efforts to remove and replace ARFTAs, drain their contents into

containers for disposal, and refill them with fresh pretreated urine are more frequent and involved than the effort to

remove and replace the higher-capacity RFTAs. The ISS program is implementing design changes to the WRS to

reduce crew time demands by enabling ARFTA servicing to be completed while installed in the rack. For

exploration missions, NASA is investing in candidate technologies to process urine brine in order to increase overall

water recovery beyond the 88% currently achieved on the ISS7. Crew time dedicated on ISS to disposing of brine

with ARFTAs could serve as a surrogate indicator for how much time might be need in exploration missions to

transfer brine to a processor, depending on the brine processing technology and system integration approach chosen.

The history of UPA and WPA ORU operating lives achieved since WRS activation is shown in histogram form

in Figures 5 through 8. For each ORU, a target operating life for each ORU is shown in black. The targets were

calculated based on supporting four crewmembers over the course of a 1000 day mission. Calculated targets also

assumed that humidity condensate and urine loads8 in an exploration transit vehicle, UPA and WPA processing

rates, and ORU duty cycles will all be comparable to those on ISS. It was also assumed that in a 1000-day

exploration mission in which outbound and return transit legs are separated by a prolonged period of surface


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exploration, expendable media items such as WPA Multifiltration Beds, Ion Exchange Bed, Particulate Filter, and

Microbial Check Valve would all be changed out prior to the return leg as part of a dormancy-management strategy.

Therefore, target lives for such expendable ORUs are assumed to be 500 days, whereas target lives for all other

electromechanical ORUs and controllers have been assumed to be 1000 days. In the histograms, the ISS predicted

life for each ORU (based on design and analysis) is shown in gray, and are labelled to indicate the basis for the

prediction (Mean Time Between Failure (MTBF) calculations, limited life component, or planned preventative

maintenance). Solid green bars represent ORUs that achieved service lives exceeding the corresponding mission

target life before being removed from service, while striped green bars indicate those that have exceeded the target

and were still in service at the time of this analysis. Solid and striped yellow bars represent ORUs that didn’t reach

the 1000 day target but for which there is reasonable confidence that the design is capable of doing so, according to

rationale discussed below. Solid and striped red bars represent ORUs that have fallen well short of the 1000 day

target.

Figure 5. UPA ORU Operating Life History Compared to a 1000-Day Mission Target Life

Of the UPA ORUs, all have achieved operating lives on the ISS that meet the 1000-day mission target life with

the exception of the FCPA. Repeated failures in the drive train linking the FCPA motor to the peristaltic pump head

have prevented any of the FCPA ORUs from achieving even half of a 1000-day target life. The same drive train

also failed in the second PCPA ORU. Development of a robust, alternative FCPA and PCPA drive train is among

the highest priority UPA design improvements that the ISS program is currently sponsoring9. The first two DA

ORUs were replaced due to failures, neither of which directly indicated a design shortcoming; the first DA failed

shortly after WRS activation due to wear induced on the ground from repeated assembly and disassembly of the

first-built unit, while the second DA failed due to calcium precipitation resulting from the unique chemistry of on-

orbit urine10. As part of the recovery from the calcium precipitation event, the first FCPA was replaced (along with

the second DA) as a precautionary measure even though it had not itself failed. As a result of this event, the ISS

program has developed an alternative urine pretreatment formulation intended to mitigate the formation of insoluble

salts at urine water recovery percentages up to 85%.11


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Of the 15 WPA ORUs, all have achieved the operating lives on the ISS that meet the 1000-day mission target

life with the exception of the Catalytic Reactor (CR) and Pump/Separator (PS). Catalytic reactor failures have all

been linked to leakage of water past o-ring seals located within the hot zones of the assembly. The ISS program has

funded design modifications to improve the sealing life in the elevated temperature environment as well as the

development of catalysts with high oxidation efficiency at lower operating temperatures which would benefit seal

durability9. The gear-style process pump within the PS ORU has failed on three separate occasions. During WPA

development, issues with the pump caused by wear of internal alumina oxide surfaces led to failures, particularly

following extended non-operating storage if the pump head was allowed to dry sufficiently to cause alumina oxide

debris to harden. The risk of lock-up following storage led to the ISS program sponsoring a modification of the

pump design to a “-2” configuration. Original “-1” pump heads were retained in the flight inventory with

procedures adopted to mitigate the risk of dryout-induced lockup during storage. The first two WPA PS on-orbit

failures occurred with the original “-1” configuration pump heads installed. Troubleshooting identified particulate

ingestion into the tightly-tolerance pump head as the cause of the first and second failures while hardened alumina

oxide prevented the third pump (“-2” configuration) from working at all. An External Filter Assembly (EFA) has

been added to the WPA upstream of the PS to protect the pump. To further protect against failures induced by solid

debris in wastewater, operation of the WPA has been modified to minimize large releases or solid biofilm or debris

from the wastewater storage tank bellows as it moves. A tightly-toleranced solenoid valve has been removed from

the flowstream, and. All remaining WPA PS ORUs include the “-2” configuration pump.

Although the WPA’s MFBs have exceeded the target life adopted in this assessment, bed replacements have

been forced by the periodic increases in potable water total organic carbon caused by dimethylsilanediol (DMSD)

migration through the WPA treatment train, rather than the saturation of MFB resins with ionic constituents per the

intended design approach12-14. Ground analyses of MFBs returned from orbit have indicated as much as 40% of

unused resin capacity remaining in the beds. Tests are currently underway9 to determine a more efficient

proportioning of resins and sorbents within the beds which, when combined with an alternate MFB changeout

strategy (that would allow initial ionic breakthrough constituents to pass to the downstream catalytic reactor) would

enable greater MFB service life for ISS or substantial reduction in MFB mass and volume for exploration.

Figure 6. WPA ORU Operating Life History Compared to a 1000-Day Mission Target Life (1 of 3)


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Figure 7. WPA ORU Operating Life History Compared to a 1000-Day Mission Target Life (2 of 3)

Figure 8. WPA ORU Operating Life History Compared to a 1000-Day Target Life (3 of 3)

V. International Space Station Oxygen Generation System

A simplified schematic of the OGA is shown in Figure 9. Feed water from the potable water bus enters the assembly

through the Water (H2O) ORU and flows through an Deionizing (DI) bed ORU, which serves to remove iodine and

coalesce gas bubbles that may be present in the feed water. If gas bubbles are detected by the gas sensor downstream

of the DI bed, the feed water is rejected to the waste water bus. The intent of this control strategy is to prevent any


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oxygen that may be present in the feed water from mixing with generated hydrogen. Water is electrolyzed into oxygen

and hydrogen in the Hydrogen ORU (H2), which contains the electrolysis cell stack. Oxygen produced by the cell

stack passes through the Oxygen Outlet ORU (O2) containing a water absorber, which protects the downstream

hydrogen sensors from liquid moisture. The Hydrogen Sensor ORU (H2S) monitors the product oxygen for the

presence of hydrogen, which would indicate a problem with the cell stack and signal the controller to shut down the

OGA. The Rotary Separator Accumulator (RSA) within the H2 ORU separates the product gaseous hydrogen from

the water which is re-circulated by the Pump ORU (P). The Nitrogen Purge ORU (N2) serves to purge system lines

upon shutdown with nitrogen. A resin bed (ACTEX-311) within the recirculation loop protects against the build-up

of harmful contaminants. The Process Controller ORU (PC) is responsible for OGA System command/control and

communication with the ISS command and data handling system. A dedicated Power Supply Module (PSM) provides

direct current to the electrolysis cell stack in proportion to the commanded oxygen production rate.

Figure 9. Oxygen Generator Assembly (OGA) Schematic

The life cycle mass associated with the OGS production of oxygen is shown in Figure 10. The initial mass of the

OGS when it was launched in 2007 was 676 kg (1487 lb). This mass includes the combined mass of the OGA ORUs

and controller, rack outfitting equipment (including an avionics air assembly, rack power control module, and smoke

detector assembly), structure (including the standard ISS rack, shelves, and brackets), and interconnecting hoses and

cables. Through March 21, 2015 the system had produced 3804 kg (8369 lb) of oxygen. Since the beginning of 2013,


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Figure 10. Oxygen Generation System Life Cycle Mass (through March 21, 2015)

the average daily oxygen production rate has been 2.34 kg/day (5.15 lb/day), corresponding roughly to a 2.9 person

oxygen production rate. Producing this amount of oxygen has required equipment to be replaced at various times

due to failures, or life limits being reached or exceeded. Since activation, 269 kg (591 lb) of equipment has been

replaced, including 170 kg (375 lb) in response to failures and 98 kg (216 lb) due to life limits. The combined

original system mass and the cumulative mass of equipment that has been replaced is 944 kg (2077 lb).

Normalizing this mass of hardware utilized with the mass of oxygen produced yields an overall equipment (original

non-recurring system plus recurring replacements) utilization mass of 0.25 kg of equipment per kg of oxygen

produced. The recurring portion of this mass utilization rate is 0.07 kg of equipment per kg of oxygen produced.

Since mid-2011, the recurring rate has been less than 0.02 kg of equipment per kg of oxygen produced.

The amount of crewtime that has been devoted to maintaining the operability of the OGS is shown in Figure 11.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000M

ass

(lb

s)

lb O2 produced

lb hardware

H2 ORU (275 lb total)

H2 Sensor (184 lb total)

H2O ORU (76 lb total)

ACTEX (27 lb total)

Pump ORU (23 lb total)

FSE (5.5 lb total)

OGS Ops Constrained by Availability of Water

Structure Cabling,

Hoses, 36%

SPOE, 5%

Controller ORUs, 6%

Tank ORUs, 3%

Process ORUs, 50%

OGS System Mass Breakdown 1486.7 lb total

Initial System Mass (lb) 1487

Hardware Changeout Mass (lb) 591

In Response to Failures (lb) 375

Expendables & Limited Life (lb) 216

Total Hardware MassTo-Date (lb) 2077

O2 Produced (lb) 8369

Hardware Mass Per O2 Produced (lb/lb) 0.25

Non-Recurring(System) + Recurring (changeout) (lb/lb) 0.25

Recurring only (lb/lb) 0.07

Hardware Statistics

Performance Statistics


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Figure 11. Crew Time Applied to Oxygen Generation System Maintenance (through March 21, 2015)

In the nearly eight years since activation, about 147 crew-hours have been recorded against OGS maintenance tasks.

When normalized against the oxygen production data shown in Figure 9, this equates to approximately 37.2 crew-

hours per 1000 kg (16.9 crew-hours per 1000 lb) of oxygen generated. Nearly two-thirds (94 crew-hours) were

dedicated to the removal and replacement of ORUs. The remaining one-third included tasks recorded as

troubleshooting (30 crew-hours), repair (12 crew-hours), inspection and servicing (5 crew-hours), cleaning avionics

air assembly filters (4 crew-hours), and removal and reinstallation (2 crew-hours).

Inspections of Figures 10 and 11 indicate that nearly 50% of the OGS life cycle mass is accounted for by the

single changeout of the H2 ORU (125 kg, 275 lb) whereas crewtime has been dominated by the repetitive

replacement of the limited life H2 Sensor ORU (30 crew-hours, representing 21% of the total crewtime dedicated to

OGS). The ISS and Advanced Exploration Systems (AES) programs have been sponsoring efforts aimed at

reducing the mass impact associated with replacement of components (such as the cell stack) currently located

within the evacuated dome of the H2 ORU as well as alternative means to detect H2 leakage across cell membranes

that don’t rely on the current H2 Sensors with their relatively short calibration life15.

The history of OGA and PSM ORU operating lives achieved since OGS activation is shown in histogram form

in Figures 12 through 15. Of the 11 ORUs that make up the OGS, all have achieved operating lives on the ISS that

meet the 1000-day mission target life with the exception of the H2 Sensor ORU. The 150-day calibration life of the

current H2 Sensor falls well short of the 1000-day target established in this study for electromechanical devices that

would not be changed out prior to return as part of a dormancy management strategy. Even if the relatively small

size of the H2 Sensor ORU (approximately 10 lb, 4.5 kg) could justify it as a planned replacement item prior to

return, its calibration life today would still need to be extended by nearly a factor of 4 to enable such a strategy.


14

Figure 12. OGA ORU Operating Life History Compared to 1000-Day Mission Target Life (1 of 3)



15


Conclusions

Operation of the ISS WRS and OGS continues to provide informative data with which to begin assessing the

technological readiness to support future missions to Mars and its vicinity. With several readily apparent exceptions,

WRS and OGS equipment has been shown to be capable of achieving operational lifetimes on the order of those

needed to support such missions. It is important to note, however, that the sample size represented by the fleet of

WRS and OGS ORUs that have been used in operational service remains very small (sample size of 1 in most cases)

and that statistical reliability predictions cannot be supported by this data alone. Furthermore, other challenges likely

to be faced in developing Mars transit and surface vehicles, such as mass and volume constraints, water and oxygen

loop closure needed to support mission scenarios, dormancy management, equipment repairability, etc., also will need

to be considered as part of an integrated Mars exploration mission and vehicle design. But in terms of highlighting

first-order trends and focus areas needing improvements, the daily operation of the ISS WRS and OGS is providing

an invaluable first step towards human Mars exploration.

Acknowledgments

The authors wish to thank Mr. Richard Mason of United Technologies Aerospace Systems and Ms. Jennifer Pruitt

of the NASA Marshall Space Flight Center for their assistance with accessing and interpreting much of the on-orbit

ORU history data presented in this paper.

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